Roaming agreements between wireless network operators enable their subscribers to use their mobile telephones when travelling in wireless networks whose operators are party to the agreement. Many wireless network operators offer mobile telephone users the option of being prepaid users. A prepaid user pays in advance for airtime and is given access to the services based on the user's account balance.
A seamless prepaid solution for users of mobile telephones roaming outside of their home mobile network requires access to each user's balance and call control capabilities by the network managing the call. Since operators are reluctant to expose their database to other networks and are missing infrastructure to manage these calls at the visited network, most operators define the prepaid users in a configuration that prevents registration outside the home mobile network.
The SICAP platform available from Swisscom of Switzerland offers prepaid service for outbound roaming. When a prepaid user is registered in a visited location register (VLR), the prepaid user is barred from making calls. To place a call, the user must type in a special Unstructured Supplementary Service Data (USSD) code, followed by the dialed number. The SICAP platform in the home mobile network receives the USSD code and generates a call back to the prepaid user and to the desired destination. This service is not seamless and forces the user to enter codes, which may be difficult to remember and time-consuming and error-prone to enter. The prepaid user's telephone book cannot be used, since a prefix of the USSD code must be added. Most significantly, the prepaid user must wait for the callback and does not receive immediate service.
In some mobile network systems, the handset has a smart card that provides some of the functionality of the handset. In Global System for Mobile communications (GSM), the smart card is called a subscriber identification module (SIM). Some mobile network operators use the SIM toolkit (STK) to produce special SIM “smart cards” which manage the prepaid balance in real time. The SIM on the handset controls the termination of calls depending on the balance stored and can recharge the balance over the air. This service is more sensitive to fraud. Furthermore, it is hard to implement complex tariff tables and hard to differentiate between roaming networks.
Some clearinghouses offer almost real-time balance update, thus enabling control over balance usage. However, there is no guarantee of the call detail record (CDR) delivery time and balance update. Delays cause unauthorized calls after the balance is exhausted.
Some mobile network operators agree to customize their network components in a particular way in order to provide roaming for prepaid users, however this solution is limited only to those operators who are party to the agreement.
The European Telecommunications Standards Institute (ETSI) has set a new GMS standard called Customized Applications for Mobile Networks Enhanced Logic (CAMEL). The standard specifies all the components needed to let GSM network operators offer the intelligent network (IN) services they supply in their home country to subscribers that roam on foreign networks. CAMEL requires an upgrade to the installed mobile network—replacing existing switches and other components with CAMEL-compliant versions. CAMEL also requires the installed mobile network to be integrated with a CAMEL-compliant service control point (SCP). This is an expensive and time-consuming procedure. Mobile network components that are compatible with CAMEL Phase 1 are commercially available and are slowly being introduced into mobile networks. CAMEL-compliant SCPs have had a less successful market penetration, due to the high price tag.
Throughout this specification and claims, the term “partially CAMEL-compliant network” is used to mean a mobile network having CAMEL-compliant versions of standard mobile network components such as switches and location registers but not having a CAMEL-compliant SCP. The term “fully CAMEL-compliant network” is used to mean a mobile network having CAMEL-compliant versions of standard mobile network components such as switches and location registers and having a CAMEL-compliant SCP.
CAMEL Phase 1 enables operators of fully CAMEL-compliant networks to offer simple telephony services to prepaid users who are roaming in a visited fully CAMEL-compliant network. CAMEL Phase 1 does not enable roaming prepaid users to receive home-language announcements or to recharge the account balance during a call without disconnecting the call. Furthermore, when at least one of the home mobile network and the visited network is not fully CAMEL-compliant, then CAMEL Phase 1 does not provide a roaming solution for prepaid users.